CORDIS Archive

Non-destructive testing - time of flight diffraction method

1. CONFORMITY WITH THE WORK PROGRAMME

This topic falls under the Competitive and Sustainable Growth Programme, generic activity Measurement and Testing. Specifically, it is related to Objective GROW-2000-6.2.1 Methodologies to Support Standardisation and Community Policies for which expressions of interest have been called.

2. KEYWORDS

Pressure equipment, manufacturing, weld inspection, Non Destructive Testing (NDT), Ultrasonic Testing (UT), Time of Flight Diffraction (TOFD), acceptance criteria, EC competitiveness, reduction of exposure dose of personnel, standardisation.

3. SUMMARY OF OBJECTIVES AND JUSTIFICATION

The present expression of interest aims at developing and favouring the industrial application of TOFD, Time Of Flight Diffraction, a non destructive testing (NDT) technique using the diffraction of ultrasonic waves by defects in materials, and specially in welds. As a second step, in compliance with the interest expressed by the CEN TC121 SC5B Chairman and EPERC TTF3 members, it is anticipated the TOFD method, testing procedure and acceptance criteria be proposed to European standardisation.

TOFD has been applied since 1980 to in-service inspection of pressure components for determining the size of defects detected by conventional NDT techniques such as radiography or conventional ultrasonic testing.

The use of TOFD for manufacturing inspection presents numerous advantages:

- more accurate sizing of weld defects,
- processing and storage of numerical data,
- health and safety of the personnel,
- increased productivity,
- protection of the environment.

However the TOFD effective industrial application as a stand-alone NDT method is hindered by the lack of European consensus on:

- the field of application of TOFD,
- the acceptance criteria,
- the lack of referential for training and certification of personnel.

The present action aims at allowing the effective application of the TOFD as a stand-alone method for the weld inspection during manufacture of pressure equipment. Technological, economical and human factors will be considered. Results will be in a suitable form for implementation in CEN standards dissemination among NDT specialists.

4. BACKGROUND

Time of Flight Diffraction (TOFD) is a non-destructive testing (NDT) technique using the diffraction of ultrasonic waves by defects in the materials, and especially in welds.

In the eighties, this method has been extensively used with success for determination of the size of defects detected in welds during in-service inspection by conventional methods such as radiography or pulse-echo ultrasound techniques. Also an application of the method for inspection of welds at the manufacturing stage has been envisaged.

Favourable results using TOFD in various national evaluation projects (Ref. 1 - 4) have demonstrated that this technique combines a high probability of detection with a low false call rate. Examples in which the technique has failed have also been reported (Ref. 5).

Methodology for applying TOFD is described in the draft European standard PrEN 583-6 prepared by TC 138.

Claims for getting agreed acceptance criteria have already been expressed (Ref. 6 - 7) and a Dutch project (Ref. 8) has already been issued.

However the application of TOFD as a routine NDT technique for welding inspection is hindered by a number of reasons:

- the performance of TOFD in comparison with conventional techniques is not generally well understood by manufacturers, engineering companies, final contractors and notified bodies;
- the performance and field of applications are not yet accepted in some European countries,
- the lack of agreed acceptance criteria;
- the discrepancies in the application of the technique;
- the lack of referential for training and certification of personnel.

The field of application and the technical limitations of the technique for defect detection shall be clearly defined and verified. Acceptance criteria related to the type of records provided by the technique shall also be proposed and assessed in term of cost and reliability.

Thus the decision to use or to not use the technique should be based on results obtained within a European project and not on the opinion of some national organisation.

5. ECONOMIC AND SOCIAL BENEFITS

As compared to traditional standardised methods, i.e. Radiographic Testing (RT) and classical Ultrasonic Testing (UT) TOFD offers definite advantages that would significantly improve EC competitiveness, environment protection and health and safety of the personnel.

As a first step, it is considered that about 30 % of the present inspection by RT or conventional UT may be substituted by TOFD in Europe.

Impact on standards

The results of this action will be used by standards on NDT welding and pressure equipment including those related to unfired pressure vessels piping and boilers.

Technico-economical benefits

- Safety and reliability of welded structures and components (inspection performances)
As all methods based on data digitalisation and processing TOFD improves inspection performances in terms of reliability, sensitivity (better defect detection and identification) and efficiency. It has the unique advantage of providing real full sizing of the defects (not possible by RT or UT) thus permitting effective cost saving in the fitness for purpose assessment (less questionable and not useful repairs).

Quality assurance
Contrary to conventional UT, TOFD provides permanent records of the results leading to a better quality management. Archiving and data storage by numerical techniques is possible for years without any risk of time dependent alteration (as it will occur for photographic films).

Archive storage volume is reduced by the use of CD-ROM or other digital mass memory devices. Document (radiography films) loss risk is suppressed (easy duplication of archived data).

- Cost savings
TOFD makes possible several cost savings and would significantly improve the EC competitiveness. As compared to RT, the inspection cost (of higher quality than radiography) will be cut by a factor of 3 due to lower cost of equipment (no use of high voltage X-ray sources or expensive radioactive sources), no manufacturing disturbances (interruption due to safety requirements), no night-working, cheap and easy data storage, etc.

- Productivity gains
At same quality level, the inspection speed with TOFD will be increased by a factor 3 as compared to conventional ultrasonic testing (UT). The productivity gain is even higher when TOFD is compared to radiography (RT) - more than 40% is expected - RT being generally performed during the night (otherwise the welding would be kept in stand-by during radiographic inspection). In addition the delay to get the inspection results will be drastically cut as compared to radiographic testing (no film processing, no film drying before inspection).

Considering inspection costs, amounts of about 10% of the whole manufacturing cost of high security pressure equipment can be saved, clearly showing that the use of the TOFD method would significantly improve the EC competitiveness.

Social benefits

· Working conditions, health and safety
The substitution of radiography by TOFD will contribute to considerable improvements of the safety of the workers. Any exposure to radiation risk is suppressed (radiography operators and neighbouring workers); no radioactive sources to handle and to store.

In addition TOFD will also contribute to the improvement of the working conditions:
- by suppressing the need for night-working generally necessary because of safety requirements;
- by improving working conditions of the operators (very uncomfortable and potentially dangerous positions of the operators may be avoided).

· Environmental impact
Replacing radiography will have a significant beneficial impact on environment by reducing the photographic effluent disposal or recycling.

· Social impact
It must be pointed out that, contrarily to the usual trend when new methods are introduced the above mentioned TOFD beneficial impacts on safety, working conditions and environment will be obtained at no job loss price. TOFD is a manual inspection method, just as radiography or conventional UT that only will need a higher training level of operators.

6. SCIENTIFIC AND TECHNOLOGICAL OBJECTIVES

General

The project shall be focused on all aspects allowing the effective application of the TOFD as a stand-alone method for the weld inspection during manufacture of pressure equipment. Technological, regulatory, human factors shall be considered. Results shall be produced in a form suitable to be implemented in CEN standards.

Scope

The project is centred on the problem of detection of defects in welds, both transverse defects (cracks) and axial defects (i.e. porosities, inclusions, lack of penetration, lack of fusion, cracks).

The project shall cover:
- non alloyed and low alloyed ferritic steels,
- butt welds (piping and plates),
- thickness range: 6 to 100 mm with emphasis on 10 to 60 mm,
- manual arc welding, TIG welding, submerged arc.
Objectives of the project

The objectives of the project are the following:

· To compare TOFD performance (both in term of acceptance criteria and best practice) with the pulse echo ultrasonic testing (UT) and the radiographic testing (RT), as applied according to the European standards defined by CEN TC 121 "Welding".

· To define the field of application of TOFD, highlighting weaknesses and strengths; if applicable, the need to use TOFD in combination with other NDT techniques shall be explained.

· To optimise the methodology of application in order to ensure reproducible inspections with different pieces of equipment and inspectors.

· To verify how TOFD allows for detection of transverse defects.

· To develop acceptance criteria.

To this aim a survey shall be performed in literature in order to review existing projects/reports. Acceptance criteria shall be defined using probabilistic fracture mechanics assessment taking account of the following:

* the repair rate must not be higher than the repair rate of the currently NDT applied methods,
* the integrity level of a structure shall be equal or better when using the developed TOFD acceptance criteria.

The acceptance criteria developed shall be validated on data collected on real structures.

The methodology used to define the criteria shall be clearly described and justified.

The acceptance criteria shall be adjusted to meet the CEN TC 121 approach as regards their differentiation in severity levels.

· To perform an economic analysis on TOFD, as defined.

· To define a framework for operators qualification and certification. Recommendations for the training and certification of the operator shall be provided and the assessment of the influence of the objectiveness of the inspector interpreting the results shall be determined.

· To disseminate the results. A spread action plan shall be established in order to disseminate in the European Union the results obtained and the proposal of guidelines. Revised guidelines taking into account the observations received shall be issued and conveyed to relevant CEN Technical Committees.

Special conditions
To achieve the objectives of the research blind trials shall be carried out in order to assess the performance, the influence of setting conditions, the reliability and reproducibility of the technique.

The blind trials shall be carried out on a set of representative welds. The justification of the adequacy of the set shall be given. The good knowledge of the real dimensions of the defects in the test pieces may require destructive examination of some test pieces or/and to perform other type of NDT for comparison purpose.

7. TIME SCALE

Although no timescale requirements apply to this project, it is expected a 3 years duration for achieving the described objectives.

8. IMPORTANT ADDITIONAL INFORMATION

As there is a need for an international agreement about the possibilities of the technique and the recognition of corresponding acceptance criteria, the European consortium in charge of research shall include experts representing at least five countries.

References

1. MG SILK and al - Defect sizing using an ultrasonic Time delay approach
   British Journal of NDT - March 1975 - Pages 33 to 36
2. D. CHAUVEAU - "Non-destructive characterisation of flaws (an account of 5 years work)
   European Journal of NDT - vol.2 Nº1 July 1992 - Pages 3 to 10
3. J. VERKOOIJEN - "The importance of reliable NDE Measurements in plant management systems"
   Proceedings of 7th ECNDT 1998 - Vol. 1 - Pages 766 to 773
4. N. TRIMBORN - "The performance of time of flight diffraction (TOFD) in various international round - robin trials and the on going research work underway today"
   Proceedings of 7th ECNDT 1998 - Vol. 2 - Pages 1231 to 1235
5. ERHARD and al. - "Critical assessment to the TOFD approach for ultrasonic weld inspection"
   Proceedings of 7th ECNDT 1998 - Vol. 2 - Pages 1236 to 1243
6. FH DIJKSTRA and al. - "Time of flight diffraction and acceptance criteria : a perfect team"
   Materials Evaluation - March 1998 - Pages 395 to 398
7. J. VERKOOIJEN and E. ZEELENBERG - "Technique performance and acceptance criteria validation for Time of Flight Diffraction (TOFD) as a pre-service inspection tool" Stoomwezen B.V. - 1998.
8. E. ZEELENBERG - "The development of acceptance criteria for the TOFD examination method"
   Final report of the Dutch joint industry research project - TO 98-54 rev.1, 1998.